Method for operating transparent node for WDM shared “virtual ring” networks

ABSTRACT

A system and method for a transparent WDM metro ring architecture in which optics enables simultaneous provisioning of dedicated wavelengths for high-end user terminals, while low-end user terminals share wavelengths on “virtual rings”. All wavelengths are sourced by the network and remotely modulated at customer “End Stations” by low cost semiconductor optical amplifiers, which also serve as transmission amplifiers. The transparent WDM metro ring architecture permits the communication of information and comprises a fiber optical feeder ring, at least one fiber optical distribution ring, a network node (NN), at least one access node (AN) said network node and said at least one access node connected via said fiber optical feeder ring and at least one end station (ES) connected via said fiber optical distribution ring to said at least one access node, wherein said user is attached to said at least one end station. A simple node that supports bi-directional propagation in transparent WDM metro architectures using “virtual rings” is also described. A method for communicating information over a WDM fiber optical ring network architecture in a metro access arena using one or more wavelengths, which can be shared by a plurality of user terminals, each user terminal coupled to an end station comprises the steps of sending downstream data packets, sending optical chalkboard packets consisting of ones and sending control signals.

This application is a continuation of prior application Ser. No.12/287,293, filed Oct. 8, 2008, which is a continuation of priorapplication Ser. No. 11/595,350, filed Nov. 9, 2006, issued as U.S. Pat.No. 7,450,846, which is a continuation of prior application Ser. No.09/902,944, filed Jul. 12, 2001, issued as U.S. Pat. No. 7,158,722 whichclaims benefit of priority of Provisional Application No. 60/217,910,filed on Jul. 13, 2000, and is related by subject matter to U.S. patentapplication Ser. No. 09/902,806 filed Jul. 12, 2001, issued as U.S. Pat.No. 7,006,767 each of which is incorporated herein by reference.

FIELD OF THE INVENTION

The invention relates to Wavelength Division Multiplexed (WDM) networksin general. More specifically, the invention relates to the use of WDMvirtual ring networks in and as metropolitan access rings where theaccess rings become transparent, may be unidirectional or bi-directionaland may also be fault tolerant.

BACKGROUND OF THE INVENTION

Optical networking has shown itself to be valuable in core transportnetworks, in large part due to the use of wavelength divisionmultiplexing (WDM). More recently, WDM optical networking has alsobecome important in the metro access arena. The D.A.R.P.A. NextGeneration Internet (NGI) initiative, for example, is especiallyinterested in investigating the issues surrounding transport of InternetProtocol (IP) traffic over such networks, and recent commercial vendoractivity in this space is also evidence of a growing awareness of theneed for flexible and high-capacity solutions.

The demands on metro networks are stringent, since the traffic is morediverse than traffic in the core, yet the system costs must be smaller.Ring architectures are generally preferred since they offer morecost-effective management and protection options, as evidenced by theperformance and acceptance of SONET systems that have been used inmetropolitan (metro) office rings. As in the core networks, WDM isexpected to play an important part for several reasons. First, it allowsexisting fiber to be “mined” for more bandwidth capacity by usingadditional wavelengths of light. This prevents “fiber exhaust” onexisting routes, defers the need to deploy more fibers, and permits moreflexible transport solutions. Traditional SONET equipment, for example,could be used on each wavelength, forestalling the need to go to higherdata rates in the hierarchy. Second, more sophisticated opticalnetworking (i.e. more sophisticated than simply increased transportcapacity) can be performed by utilizing the wavelengths as opticalchannels, which can be provisioned, added, dropped, routed, and managedas individual entities, independent of the data format they carry. Athird, and corollary reason is that WDM allows service transparency,permitting new services with independent formats to be developed anddistributed without hardware or facility changes. The extra dimension inwavelength also permits efficient and cost effective terminal solutionsthrough using transparency to transport data in native format, ratherthan requiring conversions and multiplexing. Transparency (with regardto optical networking) signifies that the optical signals do not undergooptical-to-electrical-to-optical conversions as they traverse thenetwork. Additional cost-effective properties include stability andpassivity.

SUMMARY OF THE INVENTION

An architecture suitable for metro access networks, which exploits theabove features, is described herein. Specifically, the architecture is aWDM ring, using individual wavelengths to provision services to ageographically diverse set of user terminals. Each wavelength forms avirtual ring and operates independently of the other rings. Thearchitecture further uses optical networking to allow user terminals toparticipate on different virtual rings. That is, neighboring userterminals could be on the same or different virtual rings, by virtue ofthe fiber optic connections to the ring nodes. User terminals can beprovisioned to share a wavelength with other user terminals, if costs orcommon channels dictate it, or could have dedicated wavelengths ifdemanded. Over time, the connections can change or new wavelengths canbe added. Each virtual ring forms a network of user terminals connectedto a common central hub. The virtual rings are independent, and cansupport packet-based traffic. Each is amenable to a variety of known ornew protocols. As illustrative examples, the use of two known MediaAccess Control (MAC) protocols is described. Optical technology permitsthe use of a standard optical unit that is not necessarily registered tothe wavelength of the user's virtual ring, and could be used to accessany fraction of a wavelength's bandwidth, up to the entire channelcapacity. A variety of ways of partitioning bandwidth is also described.While a single architecture is presented, it is possible to considerthis as an overlay. That is, all the wavelengths described herein can beconsidered to be some subset of the wavelengths carried on the ring: theother wavelengths might be bearing more conventional circuit-switchedtraffic, for example.

Since all of the wavelengths are centrally sourced at a common networknode, user terminals employ modular wavelength- andpolarization-independent modulators to encode upstream data onnetwork-provided optical carriers, thus alleviating much of thecomplexity related to monitoring and controlling wavelengths injectedonto the ring by user terminals. A transparent bi-directional accessnode using existing (and potential future) MAC protocols furtherimproves network efficiency. Further novel features relating toprotection against fiber and node failures, access node and end stationdesign; and improvements to the MAC protocol are also possible.

It is, therefore, an object of the present invention to allow aplurality of user terminals (each user attached to an end station) toshare one or more wavelengths with other user terminals on the virtualrings.

It is a further object of the present invention to allow multiple userterminals with arbitrary geographical distribution to share a metro ringarchitecture capable of supporting multiple virtual transparent rings.

It is yet another object of the present invention to include abi-directional node for the WDM shared virtual ring networks.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is best described with reference to the detaileddescription and the following figures, where:

FIG. 1 is a layout of the architecture.

FIG. 2 a depicts the details of an exemplary embodiment of an EndStation (ES).

FIG. 2 b depicts an alternative exemplary embodiment of an ES.

FIG. 3 depicts a Shared Virtual Ring.

FIG. 4, like FIG. 3, depicts a Shared Virtual Ring but in FIG. 4 onlytwo ESs share the bandwidth of each of two virtual rings.

FIG. 5 shows a high capacity network.

FIG. 6 a depicts two Access Nodes (ANs), each with a single ES using asingle wavelength virtual ring.

FIG. 6 b shows the BER results for back-to-back, first ES modulatedalone, and second ES modulated alone using the configuration depicted inFIG. 6 a. The inset shows an eye diagram after transmission through thesystem.

FIG. 7 a shows Two ANs, each with two ESs using a single wavelengthvirtual ring, with ESs both on the same AN, and on a remote AN.

FIG. 7 b shows the BER results using the configuration depicted in FIG.7 a, which essentially demonstrates that there is no penalty formodulation at any location.

FIG. 8 shows two virtual rings on two wavelengths, one ES per AN pervirtual ring.

FIG. 9 shows the BER results for the configuration depicted in FIG. 8,confirming essentially identical results to FIG. 6 b and FIG. 7 b.

FIG. 10 shows the packet format in the ring with alternating data andoptical chalkboard.

FIG. 11 is the eye diagram for packet experiment shows slight distortiondue to the optical chalkboard.

FIG. 12 a depicts an exemplary embodiment of a bi-directional node of ashared transparent WDM “virtual ring” network.

FIG. 12 b depicts an alternative exemplary embodiment of abi-directional node of a shared transparent WDM “virtual ring” network.

FIG. 12 c shows the BER results for the bi-directional node of theshared “virtual ring” network depicted in FIG. 12 a.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a schematic of the basic layout. In such a unidirectionalarchitecture a hub “Network Node” (NN) provides the ring's opticalcarriers, sending (1) downstream data packets, (2) “optical chalkboard”packets consisting of, for example, all “1's,” and (3) control bits orsignals. A “optical chalkboard may also consist of alternating “1's” and“0's” or any other pattern that is recognizable for the purposes ofcontrol signals, maintaining or establishing timing or relaxing thelow-frequency performance requirements of the lasers, detectors andassociated electronic circuitry. In the case of on-off keyed modulationit is desirable to have those bits that are available to have the levelof a digital one. In this way, a digital one can be created by passingand amplifying the light for one bit period. Similarly, a digital zerocan be created by blocking light with the modulator for one bit period.If the optical chalkboard consists of all “0's” then there is no opticalcarrier to be modulated or amplified. User terminals read data packetsaddressed to them, pass (through, for example, SOAs or amplifiersexhibiting substantially equivalent performance) packets not addressedto them, and write data (by gating an SOA or other type of opticalmodulator) onto optical chalkboards when permitted to do so by thecontrol bits. SOAs provide a modular, cost-effective replacement forwavelength-specific lasers by virtue of their insensitivity towavelength (over the usable optical bandwidth of the device) andpolarization variations.

The Network Node (NN), comprising WDM sources and receivers, sends WDMsignals (λ₁-λ_(N)) along the feeder fiber to Access Nodes (ANs),implemented in this embodiment with Waveguide Grating Routers (WGR).Pairs of WGR output ports define distribution loops in which a singlewavelength, forming a virtual ring, can be accessed by one or more EndStations (ES). The WGRs (also known as Arrayed Waveguide Gratings (AWGs)in the above embodiment act as static Optical Add-Drop Multiplexers(OADMs) That is the WGRs demultiplex the ring wavelengths that aredirected to users (or expressed for an AN) and then multiplex thewavelengths back onto the ring fiber. This functionality could also beperformed by other types of static or dynamic OADMs. In one embodiment,End Stations consist of receivers (Rx) for downstream information and ina preferred embodiment semiconductor optical amplifiers (SOA), whichamplify and modulate light to create upstream signals. That is, thearchitecture is a hubbed ring with a Network Node (NN) connected via“feeder” fiber to several Access Nodes (AN), which in turn servesubscribers at End Stations (ES) over “distribution” fibers. Further, asshown in FIG. 1 in an alternative ES embodiment, an ES consists ofreceivers (Rx) for downstream information and polarization independentmodulators. In the present invention, the optical signals emanate fromthe NN, are dropped optically at each AN, traverse each user ES (stillin optical form), are added onto the ring fiber optically and finallyterminate at the NN. More conventional optical networks, such as SONETring optical networks, terminate wavelengths at NNs, add and drop datain electrical form and then source new wavelengths destined for the nextnode.

The distribution scheme is based on an earlier “RITE-Net” WDM stararchitecture for passive optical networks (PONs). In that PON,individual user terminals communicate to the hub by usingwavelength-independent modulators to impress data on the opticalcarriers provided to them by the network node. The present inventionextends that architecture in several dimensions, as described below.

First, the present invention applies the distribution scheme to a ring,rather than a PON star. This is important because rings are inherentlydifferent than star configurations. This maps the PON's hub or centraloffice to the ring's Network Node and maps the PON's remote node to thering's Access Node. The NN contains (see inset, FIG. 1) a WDMtransmitter (represented as multiple sources and a multiplexer) tocreate data packets at the λ_(i) to be sent downstream for each virtualring, and a corresponding WDM receiver to detect each of the upstreamsignals. The Access Node is implemented by using a waveguide gratingrouter. Light from the feeder ring (left top fiber in AN inset) enters awaveguide grating router (WGR) input port and is demultiplexed accordingto its wavelength. According to the “routing property or scheme” of theWGR, if light of a given wavelength, after exiting a WGR port on thedistribution side, re-enters an adjacent distribution side port (λ₁ andλ₂ loopbacks in AN inset), it will emerge on a port adjacent to thefeeder ring input (right top fiber in AN inset). In this way, the ANprovides (to the distribution loops) access to the virtual rings in thefeeder fiber.

The light on the distribution loops provides both downstream informationand the ability to send upstream information, as shown in FIG. 2 a.Incoming light consists of data packets and unmodulated “opticalchalkboards” upon which upstream data signals can be impressed fortransmission. The passive splitter taps a portion of the light for thereceiver to decode downstream information (packets), and passes theremainder to the SOA. If correct permission is granted in the controlbits preceding a chalkboard (by use of for example, a protocol), the ESmay write data (an upstream message) by modulating the chalkboard. Thatis, modulated light enters the End Station for user N, where a portionis split off to a receiver, which recovers the downstream information,and the remainder is passed to a semiconductor amplifier(SOA)/modulator. The light contains both downstream data and longunmodulated portions (this is the optical chalkboard) upon, whichupstream data can be impressed by modulation. A protocol establishes,through control bits, whether or not user N can seize this “chalkboard”for modulating upstream data. The light, modulated or not, re-enters theaccess node and continues onto the feeder ring towards the next accessnode or the hub. This technique is a generalization of the PONarchitectures and, unlike an earlier approach does not require twoseparate wavelengths per user for reading and for writing data, therebysimplifying the optical infrastructure. In an alternative embodiment,the passive splitter taps a portion of the light for the receivers todecode downstream information and passes the remainder to a polarizationindependent modulator.

Second, instead of having each user, by virtue of the star topology,consume a unique wavelength, the wavelengths can be shared among eitherseveral end stations at an Access Node or among end stations atdifferent Access Nodes, or both. This permits a group of user terminalsto share bandwidth by using packets, and requires a MAC protocol todetermine which user terminals are permitted to overwrite the chalkboardin which time slots. In FIG. 1, “ES K_(i)” is meant to denote that theend station is the K^(th) end station on the virtual ring served bywavelength λ_(i). Note that in the Figure, λ₁ serves J end stationsassociated with Access Node 1 and one end station associated with AccessNode 2, so these J+1 end stations are all on the same virtual ring. Somehigh-end user terminals may consume an entire wavelength themselves.Even in this dedicated wavelength situation, there are advantages to theapproach of the present invention, since, for example, the wavelengthcontrol problem does not need to be solved by the End Station.

Third, instead of a simple modulator as in the PON, a semiconductoroptical amplifier (SOA) to both optically amplify the signals and tomodulate optical chalkboards with upstream data as shown in the EndStation inset is used. Optical amplification is necessary to support thering lengths (on the order, for example, of 200 km) expected in somemetro applications and to overcome excess losses incurred by traversingsuccessive WGRs in the Access Nodes. When the light entering the SOA isbearing data for this or another end station, the SOA acts simply as asingle channel optical amplifier. However, when data is to betransmitted upstream from the ES, the upstream electrical signal isapplied to the SOA, thereby modulating the optical chalkboard. Like abroadband optical modulator, the SOA is essentially indifferent to thewavelength (wavelength independent) of the light used: the stimulatedemission will be at the same wavelength as the input light. This featuremakes it possible to have a universal design so that each ES would beidentical, regardless of which virtual ring it occupied. Amplificationat the end stations used only as channel equalizers are used tocompensate for the loss in the fiber optical distribution loop and theassociated optical components. This avoids the need for individualamplifiers for each wavelength at each AN. In this way, the lineamplifiers can be shared over all wavelengths. As in all cascadedsystems, the buildup of noise must be considered. While the limitationimposed by amplified spontaneous emission is dependent on the lossbetween amplifiers and the amplifier noise figure, it is reasonable toexpect that a properly engineered system could support at least eightcascaded ESs on each virtual ring.

The examples in FIGS. 3-5 show the flexibility in provisioning that thearchitecture enables due to the devices. In all cases, the shaded planerepresents the physical layout of a simple network: two ANs, each withtwo user terminals nearby. In the vertical direction, different virtualrings for several wavelengths are plotted.

Thus, in FIG. 3, all four user terminals are on the same shared virtualring. The shaded plane represents the physical layout, in which two ANs,each with two ESs, share a single virtual ring. The equivalent opticalconnections are shown in the virtual ring above the plane. That is, theuser terminals served by each AN are connected on a single loop. Byvirtue of the operation of the ANs, this makes all four user terminalsconnected on the same virtual ring. The upper plane shows the connectionat λ₁, demonstrating the virtual ring directly. This is a lowperformance case in which multiple user terminals will all share theirsingle wavelength's bandwidth, according to their protocol. An examplewould be a network of remote campus locations. This is clearly a case,which exploits the fact that fiber can support multiple wavelengthssimultaneously, and permits a service, which might be difficult tojustify economically if conventional transport solutions were required,but would be an additional service opportunity as an overlay.

FIG. 4 shows a slightly more aggressive implementation. Now, twonetworks are formed, one for user terminals 1 and 4 and the other foruser terminals 2 and 3. The optical hardware is essentially the same inthis case, but now the architecture at the ANs looks as if it is in astar configuration locally, while it is still in a virtual ringconfiguration globally. Since each user on each AN is unique, thephysical layout at each AN looks more like a tree. The reduced sharingindicates that the traffic loads are higher in this case.

An even more aggressive situation is shown in FIG. 5. Applicationsrequire almost all of the line rate, so user terminals generally do notshare bandwidth with other user terminals. Note that it is possible thata single user can use more than one wavelength, and thus be on more thanone virtual ring. For example, user terminals 2 and 3 consume so muchbandwidth that they are unable to share it with other user terminals. Atthe same time, user 4, perhaps a computer facility, is both a solitaryuser on λ₄, communicating to remote sites, and also shares a ring withuser 1, perhaps an administration center, on λ₁.

This approach has some potential pitfalls, following from the fact thatthe virtual rings are unbroken light paths, which can be shared over awide area with terminal equipment that is not wavelength-specific.First, re-provisioning is more complicated than if user terminals wereconnected to a physical star architecture. Second, because the virtualring is continuous, it must be linked through ANs regardless of thenumber of End Stations served by that AN. All virtual rings not servingEnd Stations on a given AN must be completed by looping thede-multiplexed wavelengths back into the WGR, as shown in the AN detailin FIG. 1. These looped-back wavelengths must be individually amplified,adding additional cost per subscriber in a sparsely populated scenario.This problem could be solved by moving the transmission amplifiers tothe ring fiber and trimming the ES gain so that the ES and distributionfiber have zero net loss (in this way, a simple fiber jumper could beused to loop-back wavelengths that are not serving an ES on a given AN).Additionally, a wavelength assignment algorithm, a common bypass scheme,or amplifiers at output nodes of the WGRs could be used to help correctthe problem. Third, because of the terminal's simplicity, there is nowavelength reuse. Thus, if ES 2 sends a packet to ES 3, that time slotis unusable for user terminals 4 through N. This entails a tradeoffbetween system simplicity and throughput. Finally, the failure of asingle ES could disrupt service for all user terminals on that virtualring, thus potentially increasing the complexity and cost of the networkprotection scheme. In summary, the disadvantages of the network aredirectly related to the fact that a single wavelength can be shared overa wide area with terminal equipment that is not wavelength-specific.

The architecture of the present invention, however, has several expectedoperational advantages. First, the wavelength independence of the endstation optical unit should reduce the installed first cost(installation or initial costs) as well as the operational burden.Wavelength control only needs to be performed at the NN, instead of ateach of the end stations. Since many multi-wavelength WDM transmittershave the feature that they tune as a comb, it is likely that wavelengthcan be controlled at the hub with a single degree of freedom. Second,since all wavelengths are sourced and terminated at a common location,management functions, which normally require telemetry, such asperformance monitoring and fault detection, should be simplified. Third,the nature of the AN makes provisioning this network extremely flexible.Fourth, the ring nature of the connection makes it possible to useprotocols that have been optimized for ring performance, as discussedhereinafter. Newer protocols, based on others that were not specificallydesigned for rings, are also possible, utilizing the ring, modulation,and packet nature of the network.

Although many previously reported medium access control (MAC) protocolscan be modified to serve as the MAC for ring network of the presentinvention, two particularly suitable candidates are Fiber DistributedData Interface (FDDI), which includes a standardized medium accesscontrol (MAC) protocol for optical ring networks, and ADAPT, a MACproposed for networks with tree topologies. The ADAPT protocol isdescribed in “A Broadband Multiple Access Protocol for S™, ATM, andVariable Length Data Services on Hybrid Fiber-Coax Networks” by B.Doshi, S. Dravida, P. Magill, C. Siller, and K. Sriram published in BellLabs Technical Journal, Vol. 1, Number 1, 1996 and incorporated hereinby reference.

In a typical FDDI implementation, the optical signal is regenerated ateach end station, requiring a separate laser diode per site. This isespecially onerous in a WDM ring, in which each transmitter must havethe correct wavelength. However, the basic concept of FDDI can bereadily applied to our network in which the Network Node (NN) providesEnd Stations (ES) with an optical chalkboard. One token is passed fromstation to station, and the station that possesses the token at a giventime transmits data as long as allowed by its counters. It will hold thetoken at most for the negotiated time duration, unless the token hasarrived earlier than scheduled (evidence that the previous stations havenot used their negotiated bandwidths) in which case a station cantransmit for a longer time. The NN also may transmit data when it holdsthe token. If it is not transmitting data, it transmits an opticalchalkboard (during which time it does not pass any incoming packets).But while it is transmitting an optical chalkboard, some incomingpackets may arrive at the NN on their way from source to destination(for instance, when ES (J+1)₁ sends a packet to ES 1 ₁ or ES J₁). Forthis reason, FDDI should be modified somewhat in the architecture of thepresent invention. For example, packets from ES (J+1)₁ to ES 1 ₁ aresent to the NN first, are stored, and then retransmitted later when thetoken arrives at the NN. This preserves the unidirectional FDDIcharacter. Another possibility is to allow transmissions in bothdirections using either two different fibers or two differentwavelengths on the same fiber. The Network Node feedscounter-propagating rings with two optical chalkboards in this case.Then, for instance, station ES (J+1)₁ transmits packets to station ES 1₁ in the clockwise direction, while station ES 1 ₁ transmits packets tostation ES (J+1)₁ in the counter-clockwise direction.

It should be noted that the network efficiency in FDDI on aunidirectional ring is lower than that of FDDI on a bi-directional ring.In FDDI on a bi-directional ring, packets traverse the ring only once.In FDDI on a unidirectional ring, however, those upstream packets thatmust pass through the hub station (Network Node) will traverse the ringtwice, otherwise the packets will traverse the ring only once. Assumingthat r/2 of the packets pass through the Network Node, where r is thefraction of traffic remaining in the ring, then packets (on average)pass (1+r/2) rings. If v_(b) denotes the efficiency of thebi-directional FDDI, and v_(u) denotes the efficiency of theunidirectional FDDI, then it holds thatv _(u) =v _(b)/(1+r/2)=2v _(b)/(2+r)so that the unidirectional case is as much as 33% less efficient thanthe bi-directional case. Also, for unidirectional FDDI, packets thattraverse the NN may experience an additional delay waiting to beretransmitted. On the other hand, a unidirectional FDDI might have afavorable optical implementation. In both proposed FDDIs the NNcontinually negotiates the bandwidth that will be used for packetsentering and leaving the metro-ring, and therefore it flexibly followsthe change in the local-to-backbone traffic ratio.

Control in ADAPT is centralized, and in the architecture of the presentinvention control would be performed by the Network Node. End Stationsuse upstream bandwidth (e.g. parts of time slots in a slotted ring) tosend requests to the NN. The NN schedules transmissions and sendsacknowledgements to End Stations by using downstream bandwidth. ADAPTcan also be implemented on unidirectional and bi-directional rings. Ifapplied on a unidirectional ring, the efficiency in ADAPT degradessimilarly to the FDDI case above.

In comparing the FDDI and ADAPT protocols, there are some trade-offs.The benefits of FDDI are twofold. Because of its simplicity FDDI can beimplemented at high bit-rates supported by developing opticaltechnology. At the same time, FDDI guarantees end stations negotiatedbandwidth and access delays satisfying the requirements of mostmultimedia applications. FDDI-II has been developed to supportisochronous circuit-switched traffic as well. An advantage of ADAPT isthat the Network Node might meet more sophisticated service requirementssince ADAPT has complete information about the traffic in the network.On the other hand, more complex processing might be a burden at veryhigh bit-rates. The optical architecture of the present invention allowsthe MAC decision to be made based on the application.

Three experimental configurations of a 120-km three-node ring wereimplemented and used to demonstrate the flexibility of the architectureand the feasibility of using SOAs as remote modulator/amplifiers. Thefirst configuration, shown schematically in FIG. 6 a, is asingle-wavelength virtual ring shared by two user terminals, with oneuser per AN serving area. The second configuration, shown in FIG. 7 a,depicts an increase in the number of user terminals per virtual ringfrom two to four. The third configuration (FIG. 8) shows the same fouruser terminals sharing two virtual rings (two user terminals per virtualring), thus permitting higher per-user data rates than in the previoussingle virtual ring configuration. The relationships between thephysical network layouts for the last two configurations, shown in FIG.7 a and FIG. 8, and their corresponding virtual ring networks are shownin FIGS. 3 and 4, respectively.

The 120-km ring employs two ANs and a NN, each separated by 40 km ofconventional single-mode optical fiber. The average loss per 40-km spanin the tested system is 8.3 dB. In the NN, multiple single-wavelengthsources (here two sources are demonstrated) are multiplexed onto thering. The launched optical power is 6 dBm per wavelength. At each AN,the light enters a (2×16)-port WGR, with 50-GHz channel spacing, isdemultiplexed, and distributed to the user End Station(s). Each ESincludes a 3-dB splitter, which directs half the light to apolarization-insensitive (<1 dB) 1.5-μm semiconductor optical amplifier(SOA) and half to a PIN-FET receiver. The SOAs are pre-biased anddirectly modulated to 100% modulation depth with a 622 Mb/spseudo-random pattern of length 2²³-1. The typical fiber-to-fiber gainis 14 dB. After traversing the ES(s), light re-enters the AN, and ismultiplexed back onto the fiber ring, via the “routing property” of theWGR as described above. At the receiver in the NN, the demultiplexer issimulated by a tunable optical bandpass filter with a 3-dB bandwidth of1.3 nm. A variable optical attenuator and in-line power meter areinserted before the PIN-FET receiver to measure sensitivities.

FIG. 6 b shows the bit-error rate (BER) performance of the associatednetwork configuration (FIG. 6 a), as measured at the Network Node. Asingle wavelength (λ₁=1544.7 nm) serves two End Stations, each connectedto a distinct AN. The open circles correspond to modulation of thenetwork-provided optical chalkboard at the first End Station (in effect,a 622-Mb/s link from the first ES to the NN, with the second ESunmodulated and therefore serving as a transmission amplifier). Thesolid triangles show performance for modulation at the second ES (inthis case, a 622-Mb/s link from the second ES to the NN, with the firstES serving as a transmission amplifier for the optical chalkboard as ittraverses AN 1). In both cases, the performance is nearly identical,with less than 0.1-dB difference in sensitivity at 10⁻⁹ BER. The powerpenalty relative to the SOA/Modulator and PIN-FET “back-to-back” (solidcircles) is less than 0.3 dB.

FIG. 7 b shows BER performance for the same 120-km ring network, forwhich one additional ES has been added to each AN (FIG. 7 a). Since BERperformance does not vary greatly from ES to ES, only the extreme casesof 622-Mb/s modulation at the first ES (triangles) and 622-Mb/smodulation at the last ES (squares) are plotted. Again, the total powerpenalty relative to “back-to-back” is less than 0.3 dB.

FIG. 8 represents a scenario in which the demand for bandwidth isincreased, resulting in the provisioning of two virtual rings (FIG. 4)on λ₁=1544.7 nm and λ₂=1549.1 nm to serve the same set of ESs shown inFIG. 7. In this case, each AN is connected (in a local sense) to itsassociated ESs in a two-wavelength distribution star configuration(rather than the distribution loops associated with each AN in FIG. 7a). Note that each virtual ring in FIG. 8 is equivalent to the virtualring in FIG. 6 a. Thus, in the absence of crosstalk between wavelengths,it can be expected the BER performance plotted in FIG. 9 to be identicalto that in FIG. 6 b. The squares represent 622-Mb/s data added at thesecond ES on λ₁ and are also indicative of BER performance with thefirst ES modulated. In this case, λ₂ was modulated at 622 Mb/s at the NNand did not cause a measurable crosstalk penalty.

The BER data plotted in FIGS. 6 b, 7 b and 9 demonstrates upstreamtransmission for one or two wavelengths. However, based on theperformance of commercially available WGRs, and on the gain bandwidth ofthe SOAs, it can be expected that a fully populated system (eightwavelengths spread over 6.4 nm for the 16-port WGRs) would also operatewithout significant crosstalk penalties. Test data also shows up to fourSOAs in cascade without appreciable penalty. It can be expected that aproperly engineered system could support at least eight ESs per virtualring. While downstream transmission (from the NN to a user's ES) has notbeen implemented, downstream transmission is not as challenging, sincethe NN transmitters can employ conventional external modulators, whichgenerally outperform SOA/modulators.

The BER data reported above was measured in conventional continuousmode, i.e. using a repeating pseudo-random bit stream continuouslyclocked at the data rate. Although this is sufficient to demonstratemany key aspects of the ring architecture (such as the performance ofthe SOAs under high-speed modulation, SOA cascadability withoutsignificant power penalty due to Amplified Spontaneous Emission (ASE)accumulation, system power margins around the ring, and tolerance tocrosstalk), continuous BER measurements are not a valid test ofburst-mode performance. While the entire system was not tested undertrue burst-mode conditions, due to a lack of both 622-Mb/s burst-modereceivers and a burst-mode bit-error rate test set capable of operatingbeyond 200 Mb/s, SOAs were modulated with packet data to test theirsuitability as burst-mode transmitters. FIGS. 10-11 are oscilloscopetraces showing the packet-modulated signal after detection on abroadband dc-coupled optical-to-electrical converter. The trace in FIG.10 shows the entire repeating pattern consisting of eight 1024-bit slots(this length was limited by the 8192-bit maximum programmable buffersize of bit-error rate test set used). Four consecutive opticalchalkboards in slots 5-8, comprising 4096 consecutive digital ones,indicate reasonable low-frequency performance of the SOA/modulator. Thefiltered burst-mode eye diagram (FIG. 11) is wide open, but showsevidence of a slight splitting of the upper rail which should result inless than 1 dB of power penalty. The dark solid portion of the upperrail is due to the long strings of ones from the optical chalkboards. Amore thorough investigation of the network's burst-mode performance,which requires burst-mode BER testing, is currently underway.

A metro ring architecture capable of supporting multiple virtualtransparent rings, each potentially shared among multiple user terminalswith arbitrary geographical distribution has been described and has beendemonstrated at a peak rate of 622 Mb/s. User terminals modulatenetwork-provided and network-controlled wavelengths with inexpensive,polarization-insensitive and wavelength-insensitive SOA/Modulators,which also serve as in-line transmission amplifiers. BER measurementswere performed in continuous mode to test the SOAs' response tohigh-speed modulation and confirmed cascaded operation in these systems.Negligible upstream power penalties were observed for threeconfigurations of a 120-km ring: a single wavelength ring serving 2 and4 user terminals, respectively, and a two-wavelength,two-user-per-wavelength configuration. Time-domain measurements haveshown that these SOAs should perform adequately when modulated inburst-mode with packet data. True packet transmission over the network,which is necessary to realize the sharing of virtual wavelength ringsamong multiple user terminals, will require the implementation of a MACprotocol, such as modified versions of the existing FDDI or ADAPTprotocols. A simple analysis showed that the network efficiency could beimproved, for both FDDI and ADAPT, by implementing a bi-directionalversion of the network of the present invention.

While the unidirectional ring architecture described above is simple, itsuffers reduced efficiency and throughput compared to a bi-directionalring architecture, as described above. Bi-directional rings are,however, subject to scattering impairments. A novel bi-directional nodeis now described.

FIG. 12 a depicts a bi-directional node of a shared “virtual ring”network of the present invention and an experimental layout, with dashedcircles representing illustrating access nodes and user terminals thatwere not experimentally realized. The hub (NN) (shaded) contains WDMtransceivers (or transmitters and receivers) for both directions, the(unshaded) circle is the experimentally realized access node, comprisinga waveguide grating router (WGR) or arrayed waveguide grating (AWG),with a user's site (ES) below the access node. The user's site comprisestwo circulators and two transceivers of the type used in theunidirectional ring, implemented as SOAs. Light in the ring traveling inthe clockwise-counter clockwise (CW-CCW) direction emerges from the WGRonto the CW-CCW line at the user's site, finds its correspondingtransceiver through a circulator, emerging on the counterclockwise-clockwise (CCW-CW) line through the other circulator. (In thisarrangement, the user accesses both propagation directions with a singlepair, versus two pairs, of distribution optical fibers to his remotelocation at the cost of extra circulators.) To avoid serious Rayleighscattering impairments, the counter-propagating wavelengths are assumedto be different. The wavelengths are separated by at least one freespectral range of the WGR at each port.

The architecture depicted in FIG. 12 b includes a single Network Node,at which all network wavelengths are sourced. The Network Node includestwo sets of WDM transmitters and receivers shown as T_(cw), R_(cw),T_(ccw), and R_(ccw), where the subscripts “cw” and “ccw” denoteclockwise and counterclockwise propagating WDM channels, respectively.This bi-directional architecture requires that each user haveindependent access to two counter-propagating wavelengths, necessitatinga redesign of the user end station. FIG. 12 b includes two versions ofthe bidirectional ES. Implementation 1 simply consists of two of theES's shown in FIG. 2 b, with the triangle representing the discretecomponents comprising the ES in FIG. 2 b. This implementation has thedisadvantage that it requires twice the distribution fiber (it can beexpected that the maximum length of a distribution fiber loop is severalkilometers) and that it requires twice the number of ports on thedistribution side of the WGR (since each user group now requires twovirtual rings). Implementation 2 shows an ES design that avoids both ofthese shortcomings. By taking advantage of the periodicity (orcyclic-frequency) property of the WGR two wavelengths, separated by theWGR's free-spectral range (FSR), will traverse the same path from thering fiber, through the WGR, over the distribution loop and back again.A coarse mux/demux pair separates the counter-propagating wavelengths atthe ES and multiplexes them back onto the distribution loop. Amultiplexer is said to have a cyclic-frequency property if amultiplicity of approximately periodically spaced wavelengths can bemultiplexed from each input fiber to an output fiber. Similarly, ademultiplexer is said to have a cyclic-frequency property if amultiplicity of approximately periodically spaced wavelengths can bedemultiplexed to each output fiber from an input fiber.

Although direct Rayleigh scattering is avoided, it is still possible fordouble Rayleigh scattering to impair transmission. For example, counterclockwise (CCW) light (λ₁) enters the appropriate transceiver, re-entersthe ring and Rayleigh scatters from the right side, accompaniesclockwise (CW) light (λ₁₇) through the CW transceiver, re-enters thering and Rayleigh scatters from the left side, re-enters the CCW path,and interferes with itself at the CCW transceiver again. A simpleanalysis shows that this double amplification/scattering introducessystem noise as the span loss approaches the Rayleigh reflection scaledby the number of FSRs used. Filtering into directional bands at thenode, avoids this, as well as direct crosstalk from the N×Nconfiguration. The 2×2N configuration of the present invention hasexplicit rejection ports for direct crosstalk (‘top’ of the WGR).

Finally in both unidirectional and bi-directional rings, protectionschemes such as BLSR/2 are possible. While unidirectional andbi-directional rings are equally efficient in terms of lightpaths, it ismuch more efficient, from the MAC protocol perspective, to use abi-directional architecture. This follows from consideration of multiplepath trajectories a packet would have to take through the hub, in theunidirectional case, if the packet were destined for an “upstream” userrather than a downstream user.

In experiments employing the bi-directional node of a shared “virtualring” network, the Network Node contains transmitters consisting ofisolated, external cavity lasers, externally modulated (LiNbO₃) with a2³¹-1 pseudorandom sequence at 2.5 Gb/s (OC-48). The launched powerswere −2 and −6 dBm. The CW and CCW traveling wavelengths are chosen tobe separated by exactly one Free Spectral Range (FSR) of the 16×16 (50GHz spacing) frequency-cyclic WGR (i.e. λ₁, λ₁₇) of the presentinvention. A 1×2 optical splitter and an optical bandpass filter areused in lieu of the WDM.

Light entering the ring in the CCW direction traverses 40 km ofconventional single-mode fiber before entering the WGR at the AccessNode (AN). Wavelengths selected for dropping exit the WGR and aredirected via optical circulators through an unmodulated semiconductoroptical amplifier (gain approximately 17 dB) and back to the WGR. TheWGR is connected to the Network Node via another 40 km of fiber, toinduce Rayleigh scattering. At the Network Node the signals are split,optically filtered, variably attenuated, monitored for power, andreceived with a commercial clock and data regenerator for bit-error-rate(BER) testing. Light travels similarly in the CW direction. Because thetwo wavelengths are separated by one FSR, their routing table isidentical for the WGR device and both directions use the same WGR ports,and hence, the same distribution fibers.

Although the demonstration is for one user, the 16-port WGR can supportup to eight bi-directional user terminals per AN as indicated in gray inFIG. 12 a, and the dotted AN's of FIG. 12 a indicate how the ring canpotentially support multiple AN's.

The results of the BER measurements are shown in FIG. 12 c. The squaresand triangles are back-to-back baseline curves for both the CCW and CWdirections, respectively. The remaining four curves represent thefollowing: the diamonds show CW signal performance under bi-directionaloperation; the inverted triangles show CW signal performance alone, withno CCW signal present; the X′s show the CCW signal performance underbi-directional operation; the crosses monitor CCW signals without CWsignals present. All six curves are virtually overlapped (˜0.3 dB totalspread) indicating that Rayleigh backscattering and crosstalk fromadjacent FSRs over these distances result in negligible penalties forthis configuration.

A bi-directional node for WDM shared “virtual ring” networks at OC-48rates has been described and tested. Other higher or lower data ratesmay be employed in other embodiments of the present invention. It wasfound that even with 40 km of fiber on each input side to the ring,scattering impairments were insufficient to cause serious performancedegradation.

It should be clear from the foregoing that the objectives of theinvention have been met. While particular embodiments of the presentinvention have been described and illustrated, it should be noted thatthe invention is not limited thereto since modifications may be made bypersons skilled in the art. The present application contemplates any andall modifications within the spirit and scope of the underlyinginvention disclosed and claimed herein.

1. A method for operation of a bi-directional network node comprising:transmitting a data packet; transmitting an optical chalkboard packetidentified as being capable of being modulated by an end station; andtransmitting a control signal identifying a particular end station thatis authorized to modulate the optical chalkboard packet.
 2. The methodof claim 1 wherein the data packet, the optical chalkboard packet, andthe control signal are transmitted via one wavelength of a wavelengthdivision multiplexer transmitter of the bi-directional network node. 3.The method of claim 1 further comprising receiving optical signalstransmitted from an end station via an access node.
 4. The method ofclaim 3 further comprising: determining a bit error rate performance ofthe optical signals.
 5. The method of claim 4 further comprising:modifying a transmission rate of the bi-directional network node basedon the determined bit error rate performance.
 6. The method of claim 1wherein the optical chalkboard packet comprises a series of bits havingalternating values.
 7. The method of claim 1 wherein the opticalchalkboard packet comprises a series of bits, each bit of the series ofbits having a value of a digital one.
 8. A method for operation of anaccess node comprising: receiving a data packet from a bi-directionalnetwork node; receiving an optical chalkboard packet from thebi-directional network node, the optical chalkboard packet identified asbeing capable of being modulated by an end station; receiving a controlsignal identifying a particular end station that is authorized tomodulate the optical chalkboard packet; and transmitting the datapacket, the optical chalkboard packet, and the control signal to theparticular end station.
 9. The method of claim 8 further comprising:transmitting data packets, optical chalkboard packets, and controlsignals not addressed to the particular end station to a next node in atransmission sequence.
 10. The method of claim 9 wherein the next nodeis another access node.
 11. The method of claim 9 wherein the next nodeis the bi-directional network node.
 12. The method of claim 8 furthercomprising: receiving a data packet from the particular end station, anoptical chalkboard packet modulated by the particular end station, and acontrol signal from the particular end station.
 13. The method of claim12 further comprising: transmitting the received data packet, theoptical chalkboard packet, and the control signal from the particularend station.
 14. A method for operation of an end station comprising:receiving a data packet from an access node; receiving an opticalchalkboard packet from the access node, the optical chalkboard packetidentified as being capable of being modulated by an end station;receiving a control signal identifying the end station as beingauthorized to modulate the optical chalkboard packet; and modulating theoptical chalkboard packet.
 15. The method of claim 14 wherein themodulating the optical chalkboard packet comprises writing data to theoptical chalkboard packet.
 16. The method of claim 15 furthercomprising: transmitting the modulated optical chalkboard packet, thedata packet, and the control signal to the access node.